The current process routes of existing hydrometallurgical for recovering base metals such as nickel, cobalt and copper, have drawbacks such as the dependence on a source of limestone near the plant, (mining, transport and limestone milling), high investment and operating costs, logistics issues for the delivery of reagents and raw materials, high water consumption, of nickel and cobalt in steps of neutralization and the need for disposal of large quantities of liquid. Currently other drawbacks that can be cited are:                Material re-working—This re-work is generated by the attempt of avoiding nickel loss during mixed hydroxide product (MHP) and neutralization step, because these operations are carried out in two stages.        Operational difficulties caused by the current techniques employed—The current options employed to make the dissolution control of iron concentrate in the technology high pressure acid leach (HPAL) (which has also high investment costs) uses selective mining and litotype segregation;        Another problem found in cases that would be soluble magnesium is serious environmental problems due to the build up of impurities in the water case and the need to dispose of effluent in water bodies,        
As is known, the HPAL technology solves in a positive manner the acid consumption and iron dissolved when this technique is used in limonite ores. Moreover, operating in extremely critical conditions of pressure and temperature (275° C.) requiring special construction materials. The HPAL is only a technical solution to limonite ores (high iron content) for fine ores, for controlling the abrasion, and then applied only to ores where it is possible to perform the enrichment of the interest metal after the step of processing the material or application of the milling process costly and not solve the problem of acid recovery in saprolite ores.
Usually, the nickel loss in the neutralization is resolved by performing this step in two stages, with in the second stage the solid precipitate is recycled to the leaching step so as not to have a high nickel loss of. On the MHP step, is made in two stages to, in the first stage have a better quality of final product that can not be contaminated with manganese and magnesium. In the second stage, the MHP contaminated is recycled to reduce the nickel losses in the process.
The use of lime for the neutralization step is an option of low cost and acceptable performance when used in systems that have pH below 1.5. Above this pH there is a considerable nickel loss due to the low reactivity of the limestone. For this reason, the process requires a second stage of neutralization with the recycle of the solid to the stage of leaching. The waste generated by the limestone use as a neutralizing creates 50% more quantity of waste than in for example the use of MgO or other products that generate products of neutralizing soluble in water.CaCO3(s)+H2SO4=CaSO4(s)+CO2(g)+H2OMgO(s)+H2SO4=MgSO4(a)+H2O
The impurities found in the MHP are MgO not due to reacted and manganese hydrolyzed. The MgO precipitate is due to low kinetics of the reaction between MgO and nickel sulphate and manganese is due to oxidation of Mn+2 to Mn+4. In normal procedures, so that there is an increase of product quality, there will be the production of MHP in two stages, where the second step returns the product to precipitate from the leaching step.
Present process routes employed for nickel extraction from high magnesium containing lateritic ores make use of sulfuric acid as leaching agent. As a result, most metals contained in the ore are co-solubilized as metal sulfates.
In such processes, it is mandatory to increase pH after leaching and thus neutralize the slurry in order to remove impurities that are also solubilized during leaching, such neutralization being normally done with limestone or milk of lime. During neutralization, iron, copper, aluminum and chrome are precipitated as hydroxides. After neutralization, it is necessary to wash such hydroxide precipitates and separate them from the leach solution, which is normally done by a series of counter-current thickeners (CCD). The overflow from the final CCD is the pregnant leach liquor containing the valuable metals nickel and cobalt.
The following operation, nickel and cobalt precipitation is normally completed by adjusting the pH of the pregnant solution by means of addition of magnesium oxide (MgO). As a result, the so-called mixed nickel-cobalt hydroxide precipitate (MHP) is formed, which, in addition to nickel and cobalt, also contains residual amounts of impurities solubilized in the leaching step. Solid MHP is then forwarded to downstream process steps to produce commercial products.
Use of limestone and milk of lime in the first neutralization step has the drawback of generation of large quantities of sulfates and hydroxides in which considerable amounts of nickel and cobalt are absorbed and trapped. Further disadvantages of use of such neutralization agents are:                Difficulties in assuring reliable sources of lime/limestone near the hydro metallurgical plant;        Need of transportation of large quantities of such neutralizing agents to the plant;        Generation of large amounts of solid effluents, mainly calcium sulfate (gypsum);        Operational problems associated with filtering, washing of gypsum, as well as gypsum disposal in tailings dams;        Formation of large quantities of magnesium sulfate containing liquid effluent.        Large consumptions of neutralization agent required when treating ore with elevated concentrations of impurities.        
Conducting a search of the state of the art, there are some technological proposals to resolve the problems mentioned above.
The document U.S. Pat. No. 3,466,144 proposes a process route to obtain nickel and cobalt by an acid leach with the following characteristics:                Pressure acid leaching process or a sulphatation to solubilize nickel and cobalt, together with other impurities. In the stage of leaching the ore size is 400 tyler mesh.        Precipitation of impurities at pH 4 using the MgO from the process, followed by a solid liquid separation to remove iron, aluminum and other impurities.        Step-wise production of MHP by the use of MgO generated in the process. In this step the pH of the solution is at least 7 and advantageosly 8.        Crystallization of magnesium sulphate solution using a crystallizer under pressure (called in the document “non evaporative”) for the crystallization of magnesium sulphate.        The crystals of magnesium sulphate are directed to the stage of thermal decomposition, where with the preferential use of sulfur, is held the stage of reduction of magnesium sulphate is generating the MgO and SO2 gas. Natural sulfur, pyrite and other minerals and basic compounds of sulfur can be used as reducing agent.        The MgO generated in this step is returned to the process in the operations of neutralization and production of MHP. The SO2 gas is routed to a sulfuric acid plant for the regeneration of the acid which returns to the process.        
The patent described above requires the reduction of the entire feed stream below a tyler mesh of 400 which would considerably increase the ore treatment required prior to pressure acid leaching or sulphatation. The use of a high pressure vessel is also required for leaching in this process which would increase the equipment costs for this process.
In this patent, there is no mention of Mn removal, which would be required to ensure that the final product is within specifications especially when treating ores containing elevated Mn contents.
The document WO 2007/035978 proposes a process route for nickel and cobalt recovery which uses heap or atmospheric leaching with sulfuric acid as a leaching agent. A technique that uses the principle of separation of ore lithotypes saprolite and limonite which would utilize the neutralizing properties of the saprolite to reduce the free acid in the neutralization step and reduce the neutralizing reagent consumption was also mentioned.
In the neutralization stage for precipitation of impurities (Fe—Al precipitation) and the stage for the recovery of nickel and cobalt by the generation of MSP (Mixed Sulfide Precipitate), MHP, by solvent extraction or ion exchange, it was stated that preferably magnesium oxide (with options to use magnesium carbonate or magnesium hydroxide) be utilized. In the next stage of the process, the same compound of magnesium, which was regenerated from the process, is used for removal of manganese, with the aid of an oxidizing agent for the total removal of manganese ions in solution.
After the step of removing manganese, it is cited that the crystallization of magnesium sulphate from the solution can be carried out by evaporation. Alternatives to the evaporation crystallization route mentioned in this document were the use of a membrane system, which would utilize the principle of reverse osmosis or the precipitation using a strong alkali (for example caustic soda, soda ash or calcium oxide). After obtaining the magnesium sulphate in solid form, the magnesium sulphate would be transferred to a stage of calcination in order to decompose the magnesium sulphate crystals resulting in the generation of magnesium oxide (with the option to generate magnesium carbonate) and SO2 gas. The SO2 gas would then be sent to an acid plant and be converted to sulphuric acid. The magnesium oxide or magnesium carbonate would return to the process stages of precipitation of iron and aluminum, nickel and cobalt recovery and precipitation of manganese as was previously stated in U.S. Pat. No. 3,466,144.
Potential disadvantages of this process include the use of evaporative crystallization due to the high energy consumption usually related to the evaporation of water, the dependence on solution concentration and potential impact of the water balance. Calcination of the magnesium sulphate would require high temperatures to decompose the magnesium sulphate in to MgO and SO2 gas, resulting in additional energy requirements for the process.
This document also cites the use of the technique of resin in pulp for recovery of nickel and cobalt using the same principles for the precipitation of iron, aluminum and manganese along with the stage of recovery of magnesium before mentioned.
The disadvantages of this process include the use of a single leaching option to treat all size fractions and types of ore which would result in further ore treatment prior to the leaching stages.
The document WO 2007/070973 proposes a procedure for the recovery of nickel and cobalt through the recovery of magnesium oxide and regeneration of sulfuric acid which are the main inputs to process. This proposed route process has the following characteristics:                Atmospheric sulfuric leaching or heap leaching for the solubilization of nickel and cobalt;        Recovery of nickel and cobalt by precipitation of MHP using MgO generated in the process;        Precipitation of manganese using MgO generated in the process and use of kelp, lime, sodium hydroxide etc.        Recovery of crystals of magnesium sulfate preferably by the technique of common ion, where sulfuric acid is added to the effluent containing magnesium sulfate, to precipitate it.        Crystallization of magnesium sulphate was completed in two steps: initial concentration of the effluent by evaporation in ponds, then the effluent is routed to the stage of crystallization using common ion effect;        After the step of obtaining crystals of magnesium sulphate, they are going to step in reducing sulfur and preferably using a reducing agent. The document mentions the possibility of using coal and oil as reducing agents for magnesium sulphate;        The SO2 gas generated during the reduction stage is directed to a sulfuric acid plant, while the MgO produced in the process is returned to the stages of precipitation of iron and aluminum, the recovery of nickel and cobalt (MHP), and the precipitation of manganese.        
The process described in the document is when working with high concentrations of magnesium sulphate in the process of effluent. This can be illustrated with the following chemical equation: 2 MgSO4+S°→MgO+3 SO2. This equation represents the reaction of the reduction process described in document WO 2007/070973. It is required to have 2 moles of sulfur present in the magnesium sulphate and additionally 1 mol of sulfur (reagent for the reduction of magnesium sulphate) to generate the MgO and SO2 that can return to the process. The sulfur added at the stage of reduction is lost during the process, or it is necessary to purge the effluent to reduce the amount of sulfate ions in solution.
The technique proposed for crystallization the common ion does not cite the need for a step to remove sulfuric acid that is co-precipitated with crystals of magnesium sulphate. Experiments show that about 5% by weight of magnesium sulphate crystals formed by this technique are composed of co-precipitated sulfuric acid.
The document FR 2,448,577, proposes a process for recovering nickel and cobalt through the following features:                Use the technique of pressure leaching using sulfuric acid sulfuric regenerated in the process;        Precipitation of impurities such as iron and aluminum through the use of MgO generated in the process;        Production of MSP containing nickel, cobalt and zinc;        Production of MSP is achieved by injection of H2S in a pressurized reactor;        Evaporation of effluent from the generation of MSP, containing essentially magnesium sulphate. The evaporation is conducted under vacuum for the generation of crystals of MgSO4.7H2O (magnesium sulfate heptahydrate);        Heating the crystals of magnesium sulphate in a furnace at 180° C. to perform the dehydration and consequently obtaining crystals of anhydrous magnesium sulphate;        Reduction of anhydrous magnesium sulphate crystals using hydrogen gas as reducing agent. The reaction of reduction generates the SO2 gas that is routed to a sulfuric acid plant and MgO;        The MgO produced is returned to the process for precipitation of impurities in the neutralization step. The MgO produced in excess is directed to a unit of agglomeration and briquetting later to be sold or stockpiled.        
A new process route is disclosed here for treating lateritic ores, especially those with elevated levels of acid-consuming impurities such as iron and magnesium. The main feature of the process is magnesium recycling and sulfur recovery in hydrometallurgical processes such as those for nickel and cobalt. For that to be accomplished, magnesium sulfate contained in pregnant leach liquors is converted to magnesium oxide and recycled back to the process in such a way that it replaces limestone/lime used in neutralization steps. In addition, magnesium oxide is also utilized for MHP generation. As a result, the above cited disadvantages of using limestone/lime in neutralization operations are avoided. Any surplus of magnesium oxide is sold as a valuable product. Furthermore, in the process here disclosed sulfuric acid is regenerated by means of magnesium sulfate crystallization, reduction to magnesium oxide and gaseous sulfur dioxide (SO2), which is forwarded to a sulfuric acid plant and recycled back to the leaching steps.